Loot Kept chooses what tiers of loot you want to see. Any loot highlighted in orange is kept, while the grey tiers are sold instantaneously.
You can change loot kept at options, and then loot kept. You can choose between keeping any type of loot, basic to legendary. The square shows if you have it selected or not. If the square is orange, you will get it. If its white, You will not get it.